1. Field of the Invention
The present invention relates to a line narrowed laser apparatus, and more particularly to a line narrowed excimer laser apparatus or a line narrowed F2 laser apparatus serving as a light source of a reduction projection exposure tool used to the manufacture of a semiconductor, wherein a spectral index value such as the spectral purity width of the laser light is controlled.
2. Related Art
Hereafter, a description will be given of respective items of conventional techniques of the line narrowed laser apparatus used as a light source of a reduction projection exposure tool.
(Light Source for Exposure)
In conjunction with trends toward finer patterning and higher integration of semiconductor integrated circuits, there has been a demand for improvement of resolution in semiconductor exposure tools. For this reason, attempts are underway to shorten the wavelength of light emitted from a light source for exposure, and a gas laser apparatus instead of a conventional mercury lamp has come to be used as the light source for exposure. As the present gas laser apparatuses for exposure, KrF excimer laser apparatuses emitting ultraviolet rays with a wavelength of 248 nm and ArF excimer laser apparatuses emitting ultraviolet rays with a wavelength of 193 nm are used. As a next-generation exposure technique, it has been conceived to apply to the ArF exposure a liquid immersion technique in which a space between an exposure lens and a wafer is filled with a liquid to change the refractive index, and the apparent wavelength of the exposure light source is thereby shortened. In the ArF liquid immersion, the apparent wavelength becomes a short 134-nm wavelength. In addition, as a next-generation light source for exposure, an F2 laser apparatus for emitting ultraviolet rays with a wavelength of 157 is a promising candidate, and there is a possibility of the F2 laser liquid immersion exposure being adopted. It is said that the wavelength is shortened down to 115 nm in the F2 liquid immersion. (Optical Elements for Exposure and Chromatic Aberration).
(Optical Elements for Exposure and Chromatic Aberration)
A projection optical system is adopted as the optical system of most semiconductor exposure tools. In the projection optical system, chromatic aberration correction is performed by combining optical elements such as lenses having different refractive indexes. At present, in the wavelength region of 248 nm to 115 nm of the laser, i.e., a light source for exposure, there are no other optical materials suitable for use as lens materials of the projection optical system than fused silica and CaF2. For this reason, a monochromatic lens of a total refraction type constituted by fused silica alone is adopted as the projection lens of the KrF excimer laser, while a partially achromatic lens of a total refraction type constituted by fused silica and CaF2 is adopted as the projection lens of the ArF excimer laser. However, since the free-running spectral widths of the KrF and ArF excimer lasers are wide at approximately 350 to 400 pm, chromatic aberration occurs if these projection lenses are used, so that the resolution declines. Accordingly, the spectral line width of the laser light emitted from the gas laser apparatus needs to be narrowed to such an extent that the chromatic aberration can be ignored. For this reason, a line narrowing module having line narrowing elements (such as an etalon and a grating) is provided in an optical resonator of the laser apparatus so as to narrow the spectral line width. (Spectral Purity Width).
(Spectral Purity Width)
The image forming performance of the exposure tool is largely affected by not only the full width at half maximum of the spectral profile of the laser light but also a tail component of the spectral profile. Accordingly, a new index value of the spectrum which is called the spectral purity width has been introduced. This spectral purity width is evaluated by the spectral width (E95) in which 95% of the total energy is concentrated.
To ensure the quality of the integrated circuit, it is required to keep this spectral purity width down to, for example, 0.5 pm or less.
(Reasons for Stabilizing the Spectral Purity Width)
However, in recent years it has come to be said that there are cases where the quality of the integrated circuits can deteriorate even if this spectral purity width is substantially narrower than the width designed for an optical system. This is described in U.S. Pat. No. 6,721,340 and JP-A-2001-267673. For this reason, this spectral purity width needs to be controlled so as to be stabilized within a predetermined allowable width.
(Conventional Techniques for Controlling Spectral Purity Width)
As techniques for controlling the spectral purity width, a method based on wavelength shifting and a method based on grating bending control have been disclosed.
Techniques for stabilizing and controlling the spectral purity width on the basis of wavelength shifting are disclosed in U.S. Pat. No. 6,721,340 and JP-A-2001-267673. JP-A-2001-267673 discloses an invention wherein a wavelength detector is provided, and a fast tuning mechanism is provided in a line narrowing unit, and wherein on the basis of the detected wavelength, the wavelength is shifted by a very small degree at a fast speed for each pulse by the fast tuning mechanism, thereby controlling the apparent spectral purity width and keeping it within an allowable range. The phrase “controlling the apparent spectral purity width” referred to herein means control whereby the central wavelength is shifted at each moment and subjected to time integration, to thereby artificially obtain a spectral purity width corresponding to a margin of the shift.
A technique for stabilizing and controlling the spectral line width (including the purity width) on the basis of grating bending control is disclosed in JP-A-2000-312048. This JP-A-2000-312048 concerns a mechanism for precisely bending a grating of a wavelength selecting element in a line narrowing module, and a grating assembly for controlling a bidirectional spectral bandwidth. A description will be given of this technique with reference to FIG. 26.
FIG. 26 shows the grating bending mechanism for controlling the spectral purity width.
A spring housing 91 is connected to one end plate 92 of two end plates 92 extending in a direction away from a line surface of a grating 90. An adjustment rod 94 is screwed into the other end plate 93 and is inserted into the spring housing 91. Further, the adjustment rod 94 is fixed to a piston 95 provided in the spring housing 91. A compression spring 96 attached between one pressure surface 91 a of the interior of the spring housing 91 and one surface of the piston 95 and a compression spring 97 attached between the other pressure surface 91 b of the interior of the spring housing 91 and the other surface of the piston 95 are present in the spring housing 91. If the adjustment rod 94 is rotated in one direction, the concave shape of the line surface of the grating 90 becomes larger (or the convex shape becomes smaller), whereas if the adjustment rod 94 is rotated in the other direction, the convex shape of the line surface of the grating 90 becomes larger (or the concave shape becomes smaller). By the use of this grating bending mechanism, the spectral line width and the spectral purity width E95 can be controlled to some extent within a certain range.
However, with the conventional technique described in the aforementioned JP-A-2001-267673, in conjunction with the control of the spectral purity width, the central wavelength also changes concomitantly. For this reason, it is difficult to independently perform the central wavelength control for allowing the central wavelength to agree with a desired value and the spectral line width control for keeping the spectral purity width within a predetermined allowable range. For this reason, the following problems occur.    (1) In the control of the central wavelength, it is desirable to perform feedback control for each pulse, but this involves complex control.    (2) In a situation in which the central wavelength is stable, the accuracy of the central wavelength control does not present a major problem. However, in a case where there is a need to dynamically control the wavelength such as when an instruction on the change of a target wavelength is given from an exposure tool, there is a possibility of affecting the accuracy of the central wavelength control.    (3) In an initial period of a burst oscillation, a chirping phenomenon occurs in which the central frequency substantially deviates.
Furthermore, with the conventional technique described in JP-A-2000-312048, if an attempt is made to control the spectral line width to a target spectral purity width E95 by controlling the grating bending; the following problems occur.    (1) The control range of the spectral purity width which is capable of maintaining a state in which the laser output is maintained is between approximately 0.4 and 0.6 pm, and the dynamic range is small. For this reason, it is only possible to set the target value of the spectral purity width E95 in the vicinity of 0.5 pm (the details of this aspect will be described later). Moreover, in cases where the range of ±0.1 pm has been exceeded due to the effect of such as a thermal load and acoustic waves, the stabilization of the spectral purity width E95 is difficult.    (2) The bending of the grading for varying the spectral purity width is greatly enlarged by a prism beam expander, the bending needs to be effected very finely on a circular arc with a long radius of curvature (e.g., several kilometers or thereabouts). If the grating cannot be bent finely, a large effect is exerted on the spectral profile. For example, there are cases where a plurality of peaks are generated.    (3) The size of the grating used in the line narrowed excimer laser apparatus for an exposure tool is very large (with a length of 200 mm to 350 mm), and the grating bending mechanism is very precise. Therefore, the grating bending mechanism is not suitable for fast control of the spectral purity width E95.
As described above, there are various problems in performing the control of the spectral purity width E95 by the waveform shifting and grating bending, and it has been difficult to effect the control of the spectral purity width E95 in a wide control range while imparting practically no effect to the control of the central wavelength.
The present invention has been devised in view of the above-described circumstances, and its object is to allow the control of the spectral purity width E95 to be performed while imparting practically no effect to the control of the central wavelength, and stabilize the spectral purity width E95.